U.S. Pat. No. 4,195,982 assigned to Western Electric Company, Inc. discloses apparatus for extruding a tube of hollow glass from a melt using a gas pressure head. Heat is applied to a cylindrical chamber having an inert atmosphere therein and containing glass to form a viscous melt. The pressure of the inert atmosphere within the chamber is increased to cause the viscous melt to flow through an annular opening in the bottom of the chamber to form a hollow glass tube.
The above-described technique has been found to be effective for extruding hollow glass tubes. However, it was found that there was a loss of the melt material (e.g., SiO.sub.2) via vaporization due to the non-equilibrium conditions that exist in the chamber during the extrusion process. A portion of the vaporized melt material deposits in the inside upper (cooler region) portion of the chamber. Such deposition of the material and attendant loss from the melt can be considerable when extruding glass over extended periods of time. Additionally, such deposits may pick up contaminants from the chamber wall or the gaseous environment therein and ultimately fall back into the melt during the extrusion process resulting in a poor quality glass tube.
Accordingly, there is a need for a technique to eliminate or substantially inhibit the formation of deposits of vaporized melt material during the gas pressure head glass extrusion process.